[PATCH] sample: bpf: syscall_tp_kern: add dfd before filename

[PATCH] sample: bpf: syscall_tp_kern: add dfd before filename

[Song Chen]

When i was writing my eBPF program, i copied some pieces of code from
syscall_tp, syscall_tp_kern only records how many files are opened, but
mine needs to print file name.I reused struct syscalls_enter_open_args,
which is defined as:

struct syscalls_enter_open_args {
	unsigned long long unused;
	long syscall_nr;
	long filename_ptr;
        long flags;
        long mode;
};

I tried to use filename_ptr, but it's not the pointer of filename, flags
turns out to be the pointer I'm looking for, there might be something
missed in the struct.

I read the ftrace log, found the missed one is dfd, which is supposed to be
placed in between syscall_nr and filename_ptr.

Actually syscall_tp has nothing to do with dfd, it can run anyway without
it, but it's better to have it to make it a better eBPF sample, especially
to new eBPF programmers, then i fixed it.

Signed-off-by: Song Chen <chensong_2000@189.cn>
---
 samples/bpf/syscall_tp_kern.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/samples/bpf/syscall_tp_kern.c b/samples/bpf/syscall_tp_kern.c
index 50231c2eff9c..e4ac818aee57 100644
--- a/samples/bpf/syscall_tp_kern.c
+++ b/samples/bpf/syscall_tp_kern.c
@@ -7,6 +7,7 @@
 struct syscalls_enter_open_args {
 	unsigned long long unused;
 	long syscall_nr;
+	long dfd_ptr;
 	long filename_ptr;
 	long flags;
 	long mode;
-- 
2.25.1

Re: [PATCH] sample: bpf: syscall_tp_kern: add dfd before filename

[Andrii Nakryiko]

On Thu, Mar 31, 2022 at 6:34 PM Song Chen <chensong_2000@189.cn> wrote:
>
> When i was writing my eBPF program, i copied some pieces of code from
> syscall_tp, syscall_tp_kern only records how many files are opened, but
> mine needs to print file name.I reused struct syscalls_enter_open_args,
> which is defined as:
>
> struct syscalls_enter_open_args {
>         unsigned long long unused;
>         long syscall_nr;
>         long filename_ptr;
>         long flags;
>         long mode;
> };
>
> I tried to use filename_ptr, but it's not the pointer of filename, flags
> turns out to be the pointer I'm looking for, there might be something
> missed in the struct.
>
> I read the ftrace log, found the missed one is dfd, which is supposed to be
> placed in between syscall_nr and filename_ptr.
>
> Actually syscall_tp has nothing to do with dfd, it can run anyway without
> it, but it's better to have it to make it a better eBPF sample, especially
> to new eBPF programmers, then i fixed it.
>
> Signed-off-by: Song Chen <chensong_2000@189.cn>
> ---
>  samples/bpf/syscall_tp_kern.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/samples/bpf/syscall_tp_kern.c b/samples/bpf/syscall_tp_kern.c
> index 50231c2eff9c..e4ac818aee57 100644
> --- a/samples/bpf/syscall_tp_kern.c
> +++ b/samples/bpf/syscall_tp_kern.c
> @@ -7,6 +7,7 @@
>  struct syscalls_enter_open_args {
>         unsigned long long unused;
>         long syscall_nr;
> +       long dfd_ptr;
>         long filename_ptr;
>         long flags;
>         long mode;

Here's what I see on latest bpf-next:

# cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_open/format
name: sys_enter_open
ID: 613
format:
        field:unsigned short common_type;       offset:0;
size:2; signed:0;
        field:unsigned char common_flags;       offset:2;
size:1; signed:0;
        field:unsigned char common_preempt_count;       offset:3;
 size:1; signed:0;
        field:int common_pid;   offset:4;       size:4; signed:1;

        field:int __syscall_nr; offset:8;       size:4; signed:1;
        field:const char * filename;    offset:16;      size:8; signed:0;
        field:int flags;        offset:24;      size:8; signed:0;
        field:umode_t mode;     offset:32;      size:8; signed:0;

This layout doesn't correspond either to before or after state of
syscalls_enter_open_args. Not sure what's going on, but it doesn't
seem that struct syscalls_enter_open_args is correct anyways.


> --
> 2.25.1
>

Re: [PATCH] sample: bpf: syscall_tp_kern: add dfd before filename

[Song Chen]

Hi,

在 2022/4/5 06:17, Andrii Nakryiko 写道:
> On Thu, Mar 31, 2022 at 6:34 PM Song Chen <chensong_2000@189.cn> wrote:
>>
>> When i was writing my eBPF program, i copied some pieces of code from
>> syscall_tp, syscall_tp_kern only records how many files are opened, but
>> mine needs to print file name.I reused struct syscalls_enter_open_args,
>> which is defined as:
>>
>> struct syscalls_enter_open_args {
>>          unsigned long long unused;
>>          long syscall_nr;
>>          long filename_ptr;
>>          long flags;
>>          long mode;
>> };
>>
>> I tried to use filename_ptr, but it's not the pointer of filename, flags
>> turns out to be the pointer I'm looking for, there might be something
>> missed in the struct.
>>
>> I read the ftrace log, found the missed one is dfd, which is supposed to be
>> placed in between syscall_nr and filename_ptr.
>>
>> Actually syscall_tp has nothing to do with dfd, it can run anyway without
>> it, but it's better to have it to make it a better eBPF sample, especially
>> to new eBPF programmers, then i fixed it.
>>
>> Signed-off-by: Song Chen <chensong_2000@189.cn>
>> ---
>>   samples/bpf/syscall_tp_kern.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/samples/bpf/syscall_tp_kern.c b/samples/bpf/syscall_tp_kern.c
>> index 50231c2eff9c..e4ac818aee57 100644
>> --- a/samples/bpf/syscall_tp_kern.c
>> +++ b/samples/bpf/syscall_tp_kern.c
>> @@ -7,6 +7,7 @@
>>   struct syscalls_enter_open_args {
>>          unsigned long long unused;
>>          long syscall_nr;
>> +       long dfd_ptr;
>>          long filename_ptr;
>>          long flags;
>>          long mode;
> 
> Here's what I see on latest bpf-next:
> 
> # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_open/format
> name: sys_enter_open
> ID: 613
> format:
>          field:unsigned short common_type;       offset:0;
> size:2; signed:0;
>          field:unsigned char common_flags;       offset:2;
> size:1; signed:0;
>          field:unsigned char common_preempt_count;       offset:3;
>   size:1; signed:0;
>          field:int common_pid;   offset:4;       size:4; signed:1;
> 
>          field:int __syscall_nr; offset:8;       size:4; signed:1;
>          field:const char * filename;    offset:16;      size:8; signed:0;
>          field:int flags;        offset:24;      size:8; signed:0;
>          field:umode_t mode;     offset:32;      size:8; signed:0;
> 
> This layout doesn't correspond either to before or after state of
> syscalls_enter_open_args. Not sure what's going on, but it doesn't
> seem that struct syscalls_enter_open_args is correct anyways.
> 

sys_enter_open is not enabled in my system somehow and i haven't figured 
out why, then i used sys_enter_openat, whose format is:

name: sys_enter_openat
ID: 647
format:
	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
	field:int common_pid;	offset:4;	size:4;	signed:1;

	field:int __syscall_nr;	offset:8;	size:4;	signed:1;
	field:int dfd;	offset:16;	size:8;	signed:0;
	field:const char * filename;	offset:24;	size:8;	signed:0;
	field:int flags;	offset:32;	size:8;	signed:0;
	field:umode_t mode;	offset:40;	size:8;	signed:0;

print fmt: "dfd: 0x%08lx, filename: 0x%08lx, flags: 0x%08lx, mode: 
0x%08lx", ((unsigned long)(REC->dfd)), ((unsigned long)(REC->filename)), 
((unsigned long)(REC->flags)), ((unsigned long)(REC->mode))

I think in this case syscalls_enter_open_args is not applicable for 
sys_enter_openat, how about we introduce a new struct specific for 
sys_enter_openat with dfd in it?

/Song

> 
>> --
>> 2.25.1
>>
>